Optimized Detection of Fission Neutrons Using Boron Coated Straw Detectors Distributed in Moderator Material

ABSTRACT

The present invention includes an apparatus and method for neutron radiation detection. The apparatus comprises combining thin walled, boron-coated straw tubes with a plastic moderator material interspersed around the tubes. The method involves using such an apparatus through application of voltage to a central wire running inside the tubes and collecting electrical pulses generated thereby.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication No. 61/183,106, filed Jun. 2, 2009, which is herebyincorporated herein in its entirety for all purposes.

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radiation detection. More particularly, theinvention relates to a method and apparatus for passive detection offissile material with some particular applications in homeland security.Even more particularly, the invention relates to portal monitors fordetecting radiation from cargo.

2. Description of the Related Art

The limited inventory and minute natural abundance of ³He gas on Earthnecessitate the adoption of new technologies for the detection ofneutrons, especially in homeland security applications, where largevolume deployments are being considered that would exhaust the entireworld supply.

The only practical source of ³He on Earth is through production of theintermediary radioactive tritium (³H) gas. Tritium decays to ³He at arate of 5.5% per year. Tritium was produced over the time frame from1955 to 1988 for use as a critical ingredient of nuclear weapons.Production ceased in the US in 1988 and likely will not resume, as thereis currently an adequate supply to sustain the diminishing nuclearweapons inventory. The current worldwide production of ³He is estimatedat 8 kiloliters per year.

US government plans to equip major seaports with large area neutrondetectors, in an effort to intercept the smuggling of nuclear materials,have precipitated a critical shortage of ³He gas. It is estimated thatthe annual demand of ³He for US security applications alone is 22kiloliters, more than the worldwide supply. This is strongly limitingthe prospects of neutron science, safeguards, and other applicationsthat rely heavily on ³He-based detectors. Clearly, alternate neutrondetection technologies that can support large sensitive areas, have lowgamma sensitivity, and low cost must be developed.

The background to the present invention and related art is bestunderstood by reference to Applicant's own prior work, including inparticularly, U.S. Pat. No. 7,002,159 B2 (the '159) entitled “BoronCoated Straw Neutron Detector” which issued Feb. 21, 2006. The '159 ishereby incorporated by reference in its entirety, for all purposes,including, but not limited to, supplying background and enabling thoseskilled in the art to understand, make and use in Applicant's presentinvention.

Applicant's other issued patents and pending applications may also berelevant, including: (1) U.S. Pat. No. 5,573,747 entitled “Method forPreparing a Physiological Isotonic Pet Radiopharmaceutical of ⁶²CU”; (2)U.S. Pat. No. 6,078,039 entitled “Segmental Tube Array High Pressure GasProportional Detector for Nuclear Medicine Imaging”; (3) U.S. Pat. No.6,264,597 entitled “Intravascular Radiotherapy Employing a Safe LiquidSuspended Short-Lived Source”; (4) U.S. Pat. No. 6,483,114 Dl entitled“Positron Camera”; (5) U.S. Pat. No. 6,486,468 entitled “HighResolution, High Pressure Xenon Gamma Rays Spectroscopy Using Primaryand Stimulated Light Emissions”; (6) U.S. Pat. No. 7,002,159 B2 (the'159) entitled “Boron Coated Straw Neutron Detector”; (7) U.S. Pat. No.7,078,704 entitled “Cylindrical Ionization Detector with a ResistiveCathode and External Readout”; (8) U.S. patent application Ser. No.10/571,202, entitled “Miniaturized ⁶²Zn/⁶²CU Generator for HighConcentration and Clinical Deliveries of ⁶²CU Kit Formulation for theFacile Preparation of Radiolabeled Cu-bis(thiosemicarbazone) Compound”;(9) U.S. patent application Ser. No. 12/483,771 entitled “Long RangeNeutron-Gamma Point Source Detection and Imaging Using RotatingDetector”; (10) U.S. Patent Application No. 61/183,106 entitled“Optimized Detection of Fission Neutrons Using Boron Coated StrawDetectors Distributed in Moderator Material”; (11) U.S. PatentApplication No. 61/333,990 entitled “Neutron Detectors for ActiveInterrogation”; and (12) U.S. Patent Application No. 61/334,015 entitled“Nanogenerator.” Each of these listed patents and applications arehereby incorporated by reference in their entirety for all purposes.

BRIEF SUMMARY OF THE INVENTION

The present invention includes an apparatus and method for radiationdetection. The apparatus comprises combining thin walled, boron-coatedstraw tube detectors with a moderator material interspersed around thetubes. The method involves using such an apparatus through applicationof voltage to a central wire running inside the tubes and collectingelectrical pulses generated thereby.

Boron coated straw tubes as disclosed in the '159 patent, as well asprior art ³He detectors, work well in detection of low energy neutrons(“slow neutrons”), but they are inefficient detectors of high energyneutrons (“fast neutrons”). Addition of a moderator to ³He detectorsprovides limited improvement. The present invention provides the optimumdetection of fast neutrons by combining the thin walls of a boron coatedstraw tube with a moderator material. The claimed arrangement requiresless travel of neutrons between detectors and less loss of neutrons inthe moderator material which results in more efficient detection.

An embodiment of the present invention is particularly useful indetection of neutron and gamma emissions from cargo arriving in variousports of entry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a depicts a cross-section of an embodiment of the detector of thepresent invention having a continuum of closely packed straws.

FIG. 1 b (prior art) depicts a traditional ³He detector.

FIG. 2 (prior art) depicts a typical detector-moderator assembly of theprior art used as a portal monitor, with a ³He tube inside apolyethylene block.

FIG. 3 depicts the simulation setup for the testing disclosed herein.

FIG. 4 depicts an embodiment of boron-coated straw (BCS) detectors inaccordance with present invention embedded in a moderator block.

FIG. 5 depicts simulated count rate of the moderated BCS detector designhaving moderator dimensions that are fixed at 305×12.7×215 cm³, but withthe number of embedded BCS detectors varying, as plotted on theabscissa. A ²⁵²Cf source emitting 20,000 n/s was simulated at 200 cmfrom the moderator face, as illustrated in FIG. 3.

FIGS. 6 a and b depict cross-sections of embodiments of the presentinvention having different configuration examples of 96 and 152 BCSdetectors (4 mm diameter) embedded in a polyethylene block.

FIG. 7 a depicts a moderator (HDPE) block used in an assembly of theprototype portal monitor which is an embodiment of the presentinvention.

FIG. 7 b depicts the end view of a prototype monitor showing 85 BCSdetectors occupying an equal number of holes in the HDPE block.

FIG. 8 a depicts the end view of a portal monitor made in accordancewith the present invention showing array of connectors in theforeground.

FIG. 8 b shows a single preamplifier connected to all BCS detectoranodes (not shown). A detector bias of 700 V is applied through the sameconnectors.

FIG. 9 is a plot of pulse height spectrum of background counts collectedin prototype monitor.

FIG. 10 is a plot of pulse height spectra collected with ²⁵²Cf source at200 cm.

FIG. 11 is a plot of pulse height spectra collected under differentgamma exposure rates.

DETAILED DESCRIPTION OF THE INVENTION

The boron straw detector of a preferred embodiment is based on arrays ofthin walled boron-coated aluminum or copper tubes (“straws”), about 4 mmin diameter, coated on the inside with a thin layer of ¹° B-enrichedboron carbide (¹° B₄C). Thermal neutrons captured in ¹° B are convertedinto secondary particles, through the ¹° B(n,α) reaction:

¹⁰B+n→ ⁷Li+α.

The ⁷Li and a particles are emitted isotropically in opposite directionswith kinetic energies of 1.47 MeV and 0.84 MeV, respectively (dictatedby the conservation of energy and momentum). Since the boron carbidelayer is very thin, on the order of 1 μm, one of the two chargedparticles will have a high probability of escaping the wall and ionizingthe gas contained within the straw. A detector consisting of a continuumof close-packed straws 1, as shown in FIG. 1 a, coated with 1-μm-thick¹⁰B₄C, offers a stopping power for neutrons equivalent to that of 2.6atm of ³He gas detector 3 (FIG. 1 b).

In addition to high abundance and very low cost of boron on Earth, theboron-coated straw (BCS) detector offers distinct advantages overconventional ³He-based neutron detectors, including sensitivity to bothneutrons and gammas rays (gamma rays are converted in the straw wallmaterial), low weight, safety for portable use (no pressurization), andlow cost. Furthermore, in imaging applications, the BCS high level ofsegmentation supports count rate capabilities and parallax-free positionencoding, both difficult to achieve in conventional ³He pressure vesseldesigns. In addition, the straw signal rise time of 45 nsec is about 20times faster than that of a large diameter ³He tube affording muchhigher rate per detection tube and far higher microphonic immunity.

In homeland security applications there is a need to detect spontaneousfission sources, such as plutonium and uranium, which may be smuggledinto large cargo containers. The energy spectrum of neutrons emitted bythese sources resembles that of ²⁵²Cf, a common laboratory source. Itsspectrum peaks between 0.5 and 1.0 MeV, with a significant tail up to 10MeV.

The detection of such high energy neutrons is commonly achieved withslow neutron detectors surrounded by a moderator, a hydrogen-richmaterial like polyethylene (C₂H₄). Thermal neutron detectors, typicallybased on neutron reactions with ¹° B, or ³He, as discussed above, havehigh detection efficiency for slow neutrons (<0.5 eV) but theprobability of capturing neutrons with energies higher than a few keV isvery low, and drops continuously with increasing energy. Fast neutronsentering the moderating material collide with hydrogen atoms, losingmuch of their energy, and they can subsequently be captured, with highefficiency, inside the detector.

Portal monitors are a useful example of a thermal detector and moderatorassembly designed to detect high energy neutrons. Widespreadinstallation at ports of entry, where standard cargo containers can bescreened efficiently and without costly delays, has been proposed and isunder evaluation. In a common design, these monitors are installed oneither side of a drive-through lane, where a potential source inside aslowly moving cargo container is always less than 2 meters away from thedetector face. The detector itself is one or two ³He tubes, pressurizedto 3 atm or less (absolute), enclosed inside a moderator box 2, as shownin FIG. 2. The dimensions of the tube are 5.08 cm diameter by 187 cmlength. The moderator box outer dimensions are 30.5×12.7×215 cm³.

In general, the design of a moderated detector requires optimization ofthe amount of moderator present: a thicker moderator can thermalize moreneutrons, but it decreases the probability that these neutrons willreach the detector, simply because the detector is now a smallerfraction of the total volume, and moreover because neutron capture inthe moderator itself is now more probable. Since the energy spectrum ofincident neutrons can differ depending on the application, the moderatormust be designed for a specific application.

Monte Carlo Simulation

We have estimated the response of the above ³He monitors in Monte Carlosimulations, implemented in MCNP5, and compare it against the responseof a new monitor design that incorporates boron-coated straw (BCS)detectors presented earlier. We show that the BCS moderator-detectorsystem allows for more efficient optimization of the moderator materialthan the ³He design.

The simulation was set up as shown in FIG. 3. Results show that for a²⁵²Cf source 4, emitting 20,000 neutrons/s (ANSI N42.43-2006 standard),at a distance of 200 cm from the detector face, the ³He-moderateddetector counts 21 cps. The simulation assumes that, in addition to thepolyethylene surrounding the detectors, neutrons thermalize and reflectfrom the concrete ground 5, and from the steel box 6 surrounding thedetector assembly. The source 4 is surrounded by 2.5 cm thickpolyethylene.

A moderated BCS assembly made in accordance with the present inventionwith the same moderator outer dimensions, requires 63 straw detectors,to achieve the same count rate of 21 cps. Each straw detector was 205 cmin length, 4 mm in diameter, and incorporating 1 μm thick ¹° B₄C.Although 4 mm straw tubes were utilized, larger tubes can be utilized inthe present invention. Preferably, straw tubes of about 25 mm or lessare utilized. The straw detectors were distributed inside the moderatorvolume as shown in FIG. 4. Other distribution patterns, uniform ornon-uniform, are within the scope of the invention and may result inhigher sensitivity (for instance a higher concentration of straws in thefront part of the moderator). As in the simulated ³He assembly, thesimulation included a steel frame and concrete ground here, as well.

The amount of ¹⁰B neutron converter present in the 63-straw assembly is2.97 grams, or 0.297 moles of ¹⁰B. The amount of ³He present in therespective design simulated above is 1.38 grams, or 0.458 moles of ³He.Accounting for the neutron absorption cross-sections of the two isotopes(3835 barn for ¹° B and 5333 barn for ³He), we conclude that with ¹° Bwe use 2.1 times less neutron converter than ³He, in this application.This benefit is due to the optimization of moderator materialsurrounding the detectors. A large number of detectors with moderatorin-between allows more neutrons to be captured, following thermalizationsince fewer neutrons are absorbed in the plastic medium betweendetectors. With the bulky ³He design, neutrons must travel longdistances in plastic after reaching thermal energies prior toencountering the detection medium. The typical plastic employed ishigh-density polyethylene (HDPE) having a density of 0.95 g/cm³ and achemical composition of 14% hydrogen, 86% carbon. Other solid materialswith a high hydrogen content, including most plastics, can also beutilized as a moderator material.

We have estimated the count rate obtained in the BCS-based portalmonitor, as a function of the number of straw detectors. Straws weredistributed with equidistant center-to-center spacing from 22.8 mm to5.11 mm and number of straws in the moderator block from 55 to 1200.Results plotted in FIG. 5, show that count rate increases linearly withthe number of straws, before it starts leveling off when the number ofstraws reaches about 200. In this linear region, the higher costassociated with a larger number of straw detectors can be easilyjustified (high cost benefit), since the rate is proportionately higher.In the region between 200 and 800 straws, the count rate stillincreases, but with a diminishing rate; in this region the cost benefitof a higher straw number is low. Finally, when there are more than 800straw detectors embedded in the moderator, the count rate efficiencydecreases even as the number of straws increases. In this region, theamount of moderator in the interstitial space between straw detectors isnot adequate to thermalize neutrons efficiently.

FIGS. 6 a and b show configuration examples with 96 and 152 BCSdetectors. The predicted count rates are 29 cps and 39 cps,respectively. The moderator thickness between neighboring straws is 12.1mm and 9.2 mm, with center-to-center spacing of 16.1 mm and 13.2 mm,respectively.

Experimental Validation

A prototype portal monitor in accordance with the present invention wasbuilt based on a design that distributes small, boron-coated straw (BCS)detectors inside a solid block of high-density polyethylene (HDPE), asdescribed earlier. The outer dimensions of the HDPE block were30.5×12.7×215 cm³ (W×D×H), which are the typical dimensions of themoderator box found inside commercial ³He based radiation portalmonitors (RPM). The HDPE block, shown in FIG. 7, has a total of 171holes, with a pitch of about 10 mm, and can accommodate an equal numberof BCS detectors. The BCS detectors were 4 mm in diameter, 200 cm long,and were lined with ¹⁰B₄C. A total of 85 BCS detectors were used for themeasurements presented here. The detectors occupied every other hole inthe HDPE block, as shown in FIG. 7.

Measurements were conducted with two different ²⁵²Cf sources, bothpurchased from Frontier Technology (FT). The sources were measured byFT, and found to be 6.03±0.18 μg on Nov. 17, 2008 for the larger source,and 1.40±0.042 μg on Feb. 21, 2002 for the smaller source. The error inthe FT measurements was ±3%. On the day of the measurements presentedhere, the source sizes were 4.11±0.12 μg and 0.164±0.0049 μg,respectively. The corresponding neutron emission rates are 9.45×10⁶ n/sand 0.377×10⁶ n/s, respectively. In all measurements, the sources wereplaced inside a pig that surrounded the source with 0.5 cm thick lead,and 2.5 cm thick polyethylene.

A ¹³⁷Cs gamma source was used to test the gamma rejection capability ofthe prototype portal monitor. The source activity was 10 mCi.

All measurements were performed inside the laboratory area of theProportional Technologies, Inc, building in Houston, Tex.

The straw walls (cathodes) of all detectors were connected together andgrounded, using an aluminum plate, shown in FIG. 7 b. Similarly theanode wires of all straws were connected together, through custom-madehigh voltage connectors, shown in FIGS. 8 a/b. The signals were read outby connecting all anodes to a single charge sensitive preamplifier (Canberra, model 2006). An external shaper (Can berra model 2022, 1 μsshaping time) and multichannel analyzer (Amptek MCA-8000) were used. Thedetectors were biased to 700 V through a 34 nF coupling capacitor.

Neutron Background. The neutron count rate was recorded in the absenceof sources. Over a period of 4278 s, a total of 7683 counts wererecorded, giving a background rate of 1.80 cps. The energy spectrum ofbackground counts is shown in FIG. 9.

Response to ²⁵²Cf neutrons: The portal monitor was lying on a laboratorybench, oriented with its long axis parallel to the ground, as shown inFIGS. 7 a/b, and with its 30-cm-long side facing the source, which wasplaced 2 m away from the monitor side facing it. Both the source and themonitor center were 110 cm above the concrete floor. The source wassupported on a tripod.

The net count rate recorded with the large ²⁵²Cf source was 13,714 cps.The net rate recorded with the small source was 566 cps. Thecorresponding sensitivities, obtained by dividing the count rate by thesource amount, were 3.34±0.10 cps/ng, and 3.45±0.10 cps/ng,respectively. The error in these measurements is due to the uncertaintyin the neutron source size, discussed earlier (±3%). The US governmentrequirement for portal monitors is 2.5 cps/ng.

The above rates can be scaled down to the size of a standard ²⁵²Cfsource that emits 20,000 n/s. The results are 29.0±0.87 cps (largesource) and 30.0±0.90 cps (small source). By comparison, thecorresponding performance of ³He-based RPM's deployed by the USgovernment is (at least) 20 cps in designs that employ a single ³Hetube, and (at least) 32 cps in designs that employ 2 tubes.

The results are summarized in Table 1. The pulse height spectracorresponding to the above measurement (large source) are shown in FIG.10.

Response to ¹³⁷Cs gammas: The portal monitor was positioned in the samemanner as described above, for the neutron measurements. The gammasource was supported on a tripod and placed 61 cm from the front side ofthe monitor, and 1 m above the concrete floor. The exposure ratemeasured at the monitor was 10 mR/hr.

Neutron counts collected with a 10 mR/hr gamma exposure, over a 3708-stime interval, totaled 6439. This corresponds to a neutron backgroundrate of 1.74 cps. Another collection was done with a 50 mR/hr exposure,resulting in a rate of 1.80 cps. Table 2 lists these results, along withthe net count rate, obtained by subtracting the neutron backgroundmeasurement of 1.80 cps. The pulse height spectra are shown in FIG. 11.

TABLE 1 Count rates measured with ²⁵²Cf sources at 2 m. BackgroundCollection Net count Net count rate rate time rate Sensitivity forstandard¹ Source size (cps) Net counts (s) (cps) (cps/ng) source (cps)Large ²⁵²Cf source 1.80 1,398,792 102 13,714 3.34 ± 0.10 29.0 ± 0.87(4.11 ± 0.12 μg) Small ²⁵²Cf source 1.80 283,470 501 566 3.45 ± 0.1030.0 ± 0.90 (0.164 ± 0.0049 μg) ¹Standard source emits 20,000 n/s.

TABLE 2 Count rates measured with ¹³⁷Cs gamma-ray source. GammaCollection Gross Net exposure rate time Gross rate count rate (mR/hr)(s) Counts (cps) (cps) 10 3708 6439 1.74 −0.06 50 4002 7215 1.80 0.00

Although the invention has been described with reference to itspreferred embodiments, those of skill in the art may appreciate fromthis description various changes and modifications which can be madethereto which do not depart from the spirit and scope of the inventionas described and claimed herein.

¹ Standard source emits 20,000 n/s.

1. A portal monitoring system for detecting neutrons emitted from cargocomprising: a plurality of thin wall boron coated straw detectors, and amoderator material interspersed between said straw detectors.
 2. Theapparatus of claim 1, wherein the interspersion of plastic material isachieved through the use of closely packed plastic tubes having aninternal diameter about 50 to 500 μm larger than the outer diameter ofthe straw detectors.
 3. The apparatus of claim 1, wherein said tubeshave a variable wall thickness to achieve a constant straw detectordensity.
 4. The apparatus of claim 1, wherein said tubes have a variablewall thickness to achieve a variable straw detector density.
 5. Theapparatus of claim 1, wherein said straw tubes are uniformly distributedwithin said moderator material.
 6. The apparatus of claim 1, whereinsaid straw tubes are non-uniformly distributed within said moderatormaterial.
 7. The apparatus of claim 1, wherein said moderator materialis HDPE.
 8. The apparatus of claim 1, wherein said straw detectors beingspaced apart from between about 5 mm to about 23 mm.
 9. The system ofclaim 1, wherein said plurality of straw detector comprises betweenabout 50 to about 1200 individual straw detectors.
 10. The system ofclaim 1, wherein said plurality of straw detector comprises betweenabout 50 to about 800 straw detectors.
 11. The system of claim 1,wherein said plurality of straw detector comprises between about 50 toabout 200 straw detectors.
 12. The system of claim 1, wherein said strawdetectors are about 200 mm in length and 4 mm in diameter.
 13. Thesystem of claim 1, wherein said system has a sensitivity of at leastabout 3.3 cps/ng.
 14. The system of claim 1, wherein said detectordetects both neutrons and gamma emissions.
 15. The system of claim 1,wherein said straw detectors are arranged to be more concentrated near aforward portion of the moderator material.
 16. A method of detectingradiation, comprising: providing a plurality of thin wall, boron-coatedstraw detectors having a plastic material interspersed between saiddetectors; applying voltage to a central wire at said detectors; andcollecting electrical pulses generated by the detectors.
 17. A devicefor detection of energetic neutrons comprising a block of high densitypolyethylene or other plastic having high hydrogen content and minimalcontent of neutron absorbing nuclei such as Nitrogen and having holespassing through the length of the block arranged in a regular array withconstant hole center to center distance and having inserted in saidholes boron coated straw detectors consisting of a thin wall tube havinga thin inner wall coating of thickness in the range of 0.5 μm to 2.0 μmand high in ¹⁰B content of at least 60% by weight and having a thinelectrically conductive wire centered and tensioned in the middle ofsaid tube and suitable high voltage electrical insulation at the ends;high positive voltage source connected to the central wire, and acollector for the electrical pulses from one end of the wire.